1. Field of the Invention
Exemplary aspects of the present invention relate to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus and an image forming method using the image forming apparatus for controlling a voltage of an electric field to transfer a toner image.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form a color image on a recording medium (e.g., a transfer sheet) based on image data using electrophotography. Thus, for example, chargers uniformly charge surfaces of image carriers. An optical writer emits light beams onto the charged surfaces of the image carriers to form electrostatic latent images on the image carriers according to the image data, respectively. Development devices supply yellow, cyan, magenta, and black toner particles to the electrostatic latent images formed on the image carriers to make the electrostatic latent images visible as yellow, cyan, magenta, and black toner images, respectively. A transfer member transfers the toner images directly from the image carriers and superimposes the toner images onto a transfer sheet conveyed on a conveyance belt in a direct transfer method to form a color toner image on the transfer sheet. Alternatively, a first transfer member transfers the toner images from the image carriers and superimposes the toner images onto an intermediate transfer member in an indirect transfer method to form a color toner image on the intermediate transfer member, and a second transfer member transfers the color toner image from the intermediate transfer member onto a transfer sheet. Cleaners clean the surfaces of the image carriers after the toner images are transferred from the image carriers. Finally, a fixing device applies heat and pressure to the transfer sheet bearing the color toner image to fix the color toner image on the transfer sheet, thus forming the color image on the transfer sheet.
In each of the direct transfer method and the indirect transfer method, the transfer member, including the first transfer member and the second transfer member, applies a transfer bias having a polarity either identical to or opposite to a polarity of the toner image to a transfer electric field generator, that is, the conveyance belt in the direct transfer method and the intermediate transfer member and the transfer sheet in the indirect transfer method, so as to generate a transfer electric field. An electrostatic attractive force or an electrostatic repulsive force generated by the transfer electric field transfers the toner image onto the intermediate transfer member or the transfer sheet.
The transfer member and the transfer electric field generator generally include a semi-conductive material whose resistance fluctuates with environmental conditions such as temperature and humidity. The resistance also changes gradually over time due to deterioration of the semi-conductive material. Accordingly, the transfer bias applied by the transfer member to the transfer electric field generator changes, resulting in decreased transfer efficiency and formation of a faulty toner image.
To address these problems, the transfer bias is adjusted to a predetermined constant voltage or a predetermined constant current by measuring a voltage or a current flowing in the transfer member contacting the transfer electric field generator or a surface potential of the transfer electric field generator, for example. However, such measurements may not be precise due to changes in speed of an image forming operation and measurement error caused by movement of the transfer electric field generator.
Obviously, such decreased transfer efficiency and its resulting formation of a faulty toner image are undesirable, and accordingly, there is a need for a technology to generate a stable transfer electric field regardless of change in resistance of the transfer electric field generator and the transfer member.